1. Field of the Invention
The present invention relates to a MOS control thyristor which is used as a power switching element. In particular, it relates to a MOS control thyristor which can be reliably turned-off and which is less susceptible to avalanche breakdown when it is turned-off under such conditions that the thyristor is connected to an inductive load.
2. Description of the Prior Art
As a kind of thyristor which can be turned-off, a gate turn-off thyristor (hereinafter referred to as a "GTO") is a widely used. However, as the GTO is so-called current-controlled element, it requires a relatively large amount of gate-driving current, etc.. So as to overcome such problems, a MOS gate thyristor has been developed, the gate of which is controlled by a voltage drive. This MOS gate thyristor has a structure in which a wide-base bipolar transistor is driven by a MOS gate, and is similar to an insulated gate type bipolar transistor (hereinafter referred to as a "IGBT") in the structure thereof.
The difference between the MOS gate thyristor and the IGBT is that, while latching of an inner parasitic thyristor is prevented in the IGBT, latching of the inner parasitic thyristor occurs in the MOS gate thyristor. Accordingly, when turning-off the MOS gate thyristor, not only the gate voltage but the anode voltage must be reversed in polarity.
In recent years, a MOS Control Thyristor (MCT) using a MOS gate for turning-on as well as turning-off has been developed. In this structure, MOSFETS for turning-on and for turning-off are installed in a p.sup.- n- p.sup.- n thyristor. Namely, on a first region of a first conductivity type (e.g. n-type) having a high impurity concentration and of low specific resistance, a second region of a second conductivity type (e.g. p-type) having high specific resistance is formed. Then, a third region of the first conductivity type is selectively formed on the surface of this second region. Further, a fourth region of the second conductivity type is selectively formed on the surface of the third region. And lastly, a fifth region of the second conductivity type and a sixth region of the first conductivity type are formed on the surface of the fourth region. And, a gate electrode is formed through gate isolation layers on a channel region which is defined respectively as a surface region between a first part of the third region and the fourth region, and as a surface region between a second part of the third region and the sixth region. Further, a cathode electrode is so formed that it contacts the fifth and the sixth region, and an anode electrode is formed on the surface of the first region.
This element operates with the cathode electrode being grounded and with voltages applied to the gate electrode and the anode electrode. For example, assuming that the first conductivity type is n-type and the second conductivity type is p-type, a p channel is formed between the fourth region of p layer and the second region of p.sup.- layer when a negative voltage is applied to the gate electrode 8 of the thyristor to turn on.
Thereby, when a negative voltage is applied to the anode electrode, holes that are formed in the p channel begin to flow from the p channel to the anode, and turn on the junction n.sup.+ /p.sup.- between the first region and the second region. Thereby, electrons flow from the n.sup.+ layer of the first region into the p.sup.- layer of the second region. The electrons pass through the p.sup.- layer of the second region and the n layer of the third region, and turn on the junction n/p.sup.+ between the third region and the fifth region. Therefore, hole injection occurs from the fifth region and turns the npnp thyristor on.
From the above, it can be seen that the conductivity is modulated within the second and third regions, and the resistance when turning-on is reduced.
When the thyristor turns-off, and if a positive voltage is applied to the gate electrode, an n channel is formed on the surface region of the fourth region defined between the n layer of the third region and the n.sup.+ layer of the sixth region. Thereby, the third region and the fifth region are at the same potential level. Accordingly, electrons injected from the first region, even if they reach the junction n/p.sup.+ between the third region and the fifth region, flow out to the cathode through the formed n channel. Thereby, the turn-off operation is completed without hole injection occurring from the fifth region.
In the MOS control thyristor aforementioned, the third region and the fifth region are basically at the same potential level when the thyristor is turned off. However, a very small potential difference .DELTA.V actually appears between the third region and the fifth region due to current flowing in the n channel and the third region. When this .DELTA.V is more than the diffusion potential difference between the third region and the fifth region, since the junction between the third region and the fifth region turns on, it is impossible to a turn-off operation.
Further, when the thyristor turns off under such condition that an inductive load (L load) is connected thereto, a voltage due to the inductive load counter electromotive force is applied to the junction between the second region and the third region as a reverse biasing voltage. Thereby, a large electric field appears at the aforementioned junction. Moreover, in case the first conductivity type is the n-type and the second conductivity type is the p-type, since the npn transistor composed of the first, second and third regions will continue to produce a constant current, the main current thereof becomes an electron current.
Generally speaking, the impact ionization rate of the electrons when applying a high electric field (higher than 10.sup.5 V/cm) is 100 to 1000 times larger than that of holes. Therefore, there is a drawback in that avalanche breakdown is liable to occur.